The aim of the proposed research is to identify and characterize DNA topoisomerase mutants in E. coli and in the yeast, Saccharomyces cerevisiae. In E. coli the search will be for mutants lacking the newly discovered enzyme, DNA topoisomerase III. In yeast, mutants lacking either the type 1 or the type 2 topoisomerase will be sought. In each case the purpose of finding a mutant is to determine the role of that particular topoisomerase in the DNA metabolism of the cell. It could be involved in one or more of the following processes: DNA replication, transcription, recombination or repair. The yeast topoisomerases are very similar to those of mammals so that an understanding of their role in the cell will have applicability to all cells. The long range goals of the project are to gain a better understanding of the replication and functions of DNA. This has possible applicability to genetic diseases and to cancer.